What this calculates
Enter a fuel efficiency in kilometres per litre, litres per 100 km, miles per US gallon or miles per UK gallon, and this shows it in all four at once, with each conversion worked through. It also places the figure on a scale of km/L from 1 to 100, each band shown in L/100 km as well, so the way the two run in opposite directions can be read at a glance.
The formula
Every conversion goes through kilometres per litre. Three of the units are a distance on a volume of fuel, so they convert by multiplying and dividing by fixed sizes:
mpg into km/L
km/L = mpg × 1.609344 ÷ litres in the gallonkm/L into mpg
mpg = km/L × litres in the gallon ÷ 1.609344The fourth, litres per 100 km, is a volume of fuel over a distance — the other way up — so it converts by dividing into 100:
The reciprocal
L/100 km = 100 ÷ km/L · km/L = 100 ÷ L/100 kmEach size comes from its definition.
The mile
1 mi = 1.609344 km (the international mile)The US gallon
1 US gal = 231 cubic inches = 3.785411784 LThe UK gallon
1 UK gal = 4.54609 L (the imperial gallon)Every size is an exact decimal. Where a km/L figure runs past ten significant figures, the working shows it to ten, marked ≈, while every result is worked out from the exact value.
Worked example
Some conversions come out exact. 10 km/L is exactly 10 L/100 km, 20 km/L is exactly 5 L/100 km, and 8 L/100 km is exactly 12.5 km/L.
When to use it, and the mistakes to avoid
Use it when a fuel efficiency is given in a unit you do not think in, when comparing a figure in km/L with one in L/100 km or mpg, and when a figure in mpg does not say which gallon it uses.
The mistakes that cost the most:
- Multiplying instead of dividing. km/L and L/100 km are reciprocals, not multiples. 15 km/L is 6.666667 L/100 km — not 1,500 or 0.15.
- Expecting equal steps to mean equal savings. Because L/100 km is 100 ÷ km/L, going from 10 km/L to 20 km/L takes the figure from 10 to 5 L/100 km, but going from 20 km/L to 30 km/L takes it only from 5 to 3.333333 L/100 km. Compare fuel use in L/100 km, where the steps add up.
- Mixing US and UK mpg. The gallons differ, so the figures do too: 40 mpg (UK) is 33.30697 mpg (US), and 1 mpg (US) is 1.20095 mpg (UK). Check which gallon a figure uses before comparing it with another.
- Expecting every digit of every result. Results are shown to seven significant figures — 15 km/L shows as 35.28219 mpg (US) — while each is worked out from the exact value.
- Entering zero or a negative figure. Zero has no equivalent, since L/100 km would be endless, and a negative figure has no meaning, so both are refused with a message instead of being converted.
FAQ
How do I convert km/L to L/100 km?
Divide 100 by the km/L figure. So 15 km/L is 6.666667 L/100 km, 10 km/L is exactly 10 L/100 km and 20 km/L is exactly 5 L/100 km. It is a division, not a multiplication, because the two measure the same thing the other way up.
How do I convert L/100 km to km/L?
The same way: divide 100 by the L/100 km figure. So 8 L/100 km is 12.5 km/L and 5 L/100 km is 20 km/L.
How do I convert mpg to km/L?
For US mpg, multiply by 1.609344, the kilometres in a mile, and divide by 3.785411784, the litres in a US gallon. So 1 mpg (US) is 0.4251437 km/L, and 40 mpg (US) is 17.00575 km/L, or 5.880365 L/100 km.
How do I convert km/L to mpg?
Multiply by the litres in a gallon and divide by 1.609344. So 15 km/L is 35.28219 mpg with US gallons, and 42.37214 mpg with UK gallons.
Is a US mpg the same as a UK mpg?
No. A UK (imperial) gallon is 4.54609 L and a US gallon is 3.785411784 L, so the same fuel efficiency is always the larger figure in UK mpg. 1 mpg (US) is 1.20095 mpg (UK), and 40 mpg (UK) is 33.30697 mpg (US).
Why can't I convert zero?
Because litres per 100 km is 100 divided by km/L, and nothing can be divided by zero. 0 km/L would be an endless number of litres per 100 km, and 0 L/100 km an endless distance on a litre. A negative figure has no meaning either, so both are refused with a message.